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Wind warns of big waves in record time

5 June 2004

DARK patches on the ocean surface could help warn coastal communities of the approach of a menacing wave.

Tsunamis are huge waves that swell out from the sites of underwater earthquakes or landslides, travelling across oceans at around 800 kilometres per hour. In deep water, they only raise the surface of the ocean by a few centimetres, but when they near a shore they billow up into giant tidal waves.

People on the shore have sometimes reported seeing mysterious strips of dark water several kilometres wide moving across the sea surface before tsunamis strike. It was unclear why, but now …